Some random thoughts about crypto. Notes from a course I teach. Pictures of my dachshunds.

Matthew Green

I'm a cryptographer and professor at Johns Hopkins University. I've designed and analyzed cryptographic systems used in wireless networks, payment systems and digital content protection platforms. In my research I look at the various ways cryptography can be used to promote user privacy.

While there’s not much new information in here, people on Twitter seem to have some renewed interest in how iMessage works; whether Apple could backdoor it if they wanted to; and whether the courts could force them to. The answers to those questions are respectively: “very well“, “absolutely“, and “do I look like a national security lawyer?”

So rather than tackle the last one, which nobody seems to know the answer to, I figure it would be informative to talk about the technical issues with iMessage (again). So here we go.

How does iMessage work?

Fundamentally the mantra of iMessage is “keep it simple, stupid”. It’s not really designed to be an encryption system as much as it is a text message system that happens to include encryption. As such, it’s designed to take away most of the painful bits you expect from modern encryption software, and in the process it makes the crypto essentially invisible to the user. Unfortunately, this simplicity comes at some cost to security.

Let’s start with the good: Apple’s marketing material makes it clear that iMessage encryption is “end-to-end” and that decryption keys never leave the device. This claim is bolstered by their public security documentation as well as outside efforts to reverse-engineer the system. In iMessage, messages are encrypted with a combination of 1280-bit RSA public key encryption and 128-bit AES, and signed with ECDSA under a 256-bit NIST curve. It’s honestly kind of ridiculous, but whatever. Let’s call it good enough.

iMessage encryption in a nutshell boils down to this: I get your public key, you get my public key, I can send you messages encrypted to you, and you can be sure that they’re authentic and really came from me. Everyone’s happy.

But here’s the wrinkle: where do those public keys come from?

Where do you get the keys?

Key request to Apple’s server.

It’s this detail that exposes the real weakness of iMessage. To make key distribution ‘simple’, Apple takes responsibility for handing out your friends’ public keys. It does this using a proprietary key server that Apple owns and operates. Your iPhone requests keys from Apple using a connection that’s TLS-encrypted, and employs some fancy cryptographic tokens. But fundamentally, it relies on the assumption that Apple is good, and is really going to give you you the right keys for the person you want to talk to.

But this honesty is just an assumption. Since the key lookup is completely invisible to the user, there’s nothing that forces Apple to be honest. They could, if inspired, give you a public key of their choosing, one that they hold the decryption key for. They could give you the FBI’s key. They could give you Dwayne “The Rock” Johnson’s key, though The Rock would presumably be very non-plussed by this.

Indeed it gets worse. Because iMessage is designed to support several devices attached to the same account, each query to the directory server can bring back many keys — one for each of your devices. An attacker can simply add a device (or a fake ‘ghost device’) to Apple’s key server, and senders will encrypt messages to that key along with the legitimate ones. This enables wiretapping, provided you can get Apple to help you out.

But why do you need Apple to help you out?

As described, this attack doesn’t really require direct collaboration from Apple. In principle, the FBI could just guess the target’s email password, or reset the password and add a new device all on their own. Even with a simple subpoena, Apple might be forced to hand over security questions and/or password hashes.

The real difficulty is caused by a final security feature in iMessage: when you add a new device, or modify the devices attached to your account, Apple’s key server sends a notification to each of the existing devices already to the account. It’s not obvious how this feature is implemented, but one thing is clear — it seems likely that, at least in theory, Apple could shut it off if they needed to.* After all, this all comes down to code in the key server.

Fixing this problem seems hard. You could lock the key server in a giant cage, then throw away the key. But as long as Apple retains the ability to update their key server software, solving this problem seems fundamentally challenging. (Though not impossible — I’ll come back to this in a moment.)

Can governments force Apple to modify their key server?

It’s not clear. While it seems pretty obvious that Apple could in theory substitute keys and thus enable eavesdropping, in practice it may require substantial changes to Apple’s code. And while there are a few well-known cases in which the government has forced companies to turn over keys, changing the operation of a working system is a whole different ball of wax.

And iMessage is not just any working system. According to Apple, it handles several billion messages every day, and is fundamental to the operation of millions of iPhones. When you have a deployed system at that scale, the last thing you want to do is mess with it — particularly if it involves crypto code that may not even be well understood by its creators. There’s no amount of money you could pay me to be ‘the guy who broke iMessage’, even for an hour.

Any way you slice it, it’s a risky operation. But for a real answer, you’ll have to talk to a lawyer.

Why isn’t key substitution a good solution to the ‘escrow’ debate?

Another perspective on iMessage — one I’ve heard from some attorney friends — is that key server tampering sounds like a pretty good compromise solution to the problem of creating a ‘secure golden key‘ (AKA giving governments access to plaintext).This view holds that key substitution allows only proactive eavesdropping: the government has to show up with a warrant before they can eavesdrop on a customer. They can’t spy on everyone, and they can’t go back and read your emails from last month. At the same time, most customers still get true ‘end to end’ encryption.

I see two problems with this view. First, tampering with the key server fundamentally betrays user trust, and undermines most of the guarantees offered by iMessage. Apple claims that they offer true end-to-end encryption that they can’t read — and that’s reasonable in the threat model they’ve defined for themselves. The minute they start selectively substituting keys, that theory goes out the window. If you can substitute a few keys, why not all of them? In this world, Apple should expect requests from every Tom, Dick and Harry who wants access to plaintext, ranging from divorce lawyers to foreign governments.

The second, more technical problem is that key substitution is relatively easy to detect. While Apple’s protocols are an obfuscated mess, it is at least in theory possible for users to reverse-engineer them to view the raw public keys being transmitted — and thus determine whether the key server is being honest. While most criminals are not this sophisticated, a few are. And if they aren’t sophisticated, then tools can be built to make this relatively easy. (Indeed, people have already built such tools — see my key registration profile at right.)

Thus key substitution represents at most a temporary solution to the ‘government access’ problem, and one that’s fraught with peril for law enforcement, and probably disastrous for the corporations involved. It might seem tempting to head down this rabbit hole, but it’s rabbits all the way down.

What can providers do to prevent key substitution attacks?

Signal’s “key fingerprint” screen.

From a technical point of view, there are a number of things that providers can do to harden their key servers. One is to expose ‘key fingerprints’ to users who care, which would allow them to manually compare the keys they receive with the keys actually registered by other users. This approach is used by OpenWhisperSystems’ Signal, as well as PGP. But even I acknowledge that this kind of stinks.

A more user-friendly approach is to deploy a variant of Certificate Transparency, which requires providers to publish a publicly verifiable proof that every public key they hand out is being transmitted to the whole world. This allows each client to check that the server is handing out the actual keys they registered — and by implication, that every other user is seeing the same thing.

The most complete published variant of this is called CONIKS, and it was proposed by a group at Princeton, Stanford and the EFF (one of the more notable authors is Ed Felten, now Deputy U.S. Chief Technology Officer). CONIKS combined key transparency with a ‘verification protocol’ that allows clients to ensure that they aren’t being sidelined and fed false information.

CONIKS isn’t necessarily the only game in town when it comes to preventing key substitution attacks, but it represents a powerful existence proof that real defenses can be mounted. Even though Apple hasn’t chosen to implement CONIKS, the fact that it’s out there should be a strong disincentive for law enforcement to rely heavily on this approach.

So what next?

That’s the real question. If we believe the New York Times, all is well — for the moment. But not for the future. In the long term, law enforcement continues to ask for an approach that allows them to access the plaintext of encrypted messages. And Silicon Valley continues to find new ways to protect the confidentiality of their user’s data, against a range of threats beginning in Washington and proceeding well beyond.

How this will pan out is anyone’s guess. All we can say is that it will be messy.

Notes:* How they would do this is really a question for Apple. The feature may involve the key server sending an explicit push message to each of the devices, in which case it would be easy to turn this off. Alternatively, the devices may periodically retrieve their own keys to see what Apple’s server is sending out to the world, and alert the user when they see a new one. In the latter case, Apple could selectively transmit a doctored version of the key list to the device owner.

Apple controls iPhones. They have root access, you don't. It's the future that Richard Stallman predicted in The Right To Read, except he thought it would be Microsoft. In this kind of situation, why would you even mention key substitution attacks? It's like speculating about the owners of an Internet café implementing a TLS MITM attack in their own network's router in order to spy on HTTPS connections originating from their own machines. Why bother when you're root?

Few thoughts:1. I've understood that when replacing iDevices, the new device gets also old iMessages. This is even possible without using the iCloud or breaking device-to-device encryption promise. Message transfer can be achieved by having at least one device with the messages to be in the network at the same time with the new one, and then forwarding the messages encrypted with the new public key. But can anyone confirm, that transfer does not work if iCloud is not in use and no old devices with the messages are not in the network at the same time (e.g. old one is broken)?2. Matthew mentioned, that it is hard to imperceptibly add new devices to iMessage account because a notification is sent to existing devices. How about if the first key added to the iMessage system is not yours? In theory this does not even break the promise of device-to-device encryption (because message is sent to all of the account's devices encrypted with their public key), you just don't own one of the devices to whom the encrypted message is sent. This would be possible to spot by monitoring what public keys the sending device fetches for encryption purposes. But as Matthew says, “the key lookup is completely invisible to the user”.

I think the swiss app Threema handles the public-key-check quite well: The keys are also distributed by a central server, but by scanning a qr-code of the partners key, you see that it's the right key you have. Sadly, only one device is supported which is not as flexible as iMessage is.

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You did an exceptional investigation. When I saw this News in Newspapers. I thought something like: “OH! Don't mind / Apple will never do so”. After reading your article, my thoughts changed to the opposite side: “How can Apple do so?”. Need to think about it…

In reference to your first question — you seem seriously confused. If I replace my iPhone, it is RESTORING from backup that brings the messages back. If my device is backed up, restoring brings back the messages. You can even examine the messages on OSX with an SQLITE viewing application.